Weigh/fill systems for dispensing measured amounts of powdered or comminuted materials demand special considerations from the standpoint of speed and accuracy of filling. Vibratory feeders are customarily employed in conjunction with a supply hopper as a source of the powdered material and which directs the material Onto one end of a shallow trough at the upper surface of a vibratory feeder. The vibratory feeder trough includes some form of reciprocal plunger which is operative to oscillate a spring-like support assembly for the trough so as to cause the material to be advanced horizontally toward the discharge end of the trough where it is deposited onto a weigh pan. Representative of commercially available vibratory feeder systems is the Model FTOC Vibratory Feeder manufactured and sold by Food Machinery Corporation of Homer City, Pa.
In U.S. Pat. No. 4,431,071, owned by the assignee of this application, there is disclosed a precision weigh/fill system for comminuted materials and which employs a vibratory feeder of the type described for the purpose of advancing the materials from the feeder trough into a rotatable weigh pan to which an unbalanced force is applied so as to cause the weigh pan to rotate through a predetermined angle to dump the contents into a tiltable discharge funnel. A weigh sensor is employed to weigh the material being fed into the pan and, depending upon accuracy or weigh tolerances, a discharge funnel is stationed beneath the weigh pan which is selectively tiltable to direct the material from the weigh pan either into a fill station or reject station. The system described has certain advantages in the respect that it is capable of sensing inaccuracies in filling without interrupting the dispensing operation as well as to minimize material losses and inaccurate fillings.
The present invention is concerned more with the accuracy and speed of fill from the vibratory feeder into a weigh pan or other receptacle. The practice of advancing powdered material horizontally along the trough and merely permitting it to drop off the end of the trough is not satisfactory either from the standpoint of speed or accuracy of filling: Certain powdered materials will tend to adhere to the end of the trough and not undergo even flow in their passage off the end of the trough. Flowability and particle size are important factors, also, and make it even more difficult to assure consistent filling. Still further, many powdered materials will tend to collect or lump together in their advancement along the trough.
It has been proposed in the past to employ programmed feed control systems to weigh the material as it is being filled and essentially employ real time feedback to establish the feeder cut-off point and to estimate the feeder cut-off point based on current readings of the weight. Thus, when the weighing system reaches a certain point ahead of a target weight, the feeder is stopped and if the system has currently estimated all related factors, such as, materials suspended in the air and weight not yet recorded due to time lag in the electronics, the final weight will be within the prescribed limits. However, it is desirable not only to minimize balance response as a factor in obtaining fill accuracy but to increase the speed and accuracy of filling by a combination of closely controlled rate of discharge of material with accurate readings of specific proportions of the total target weight at intervals throughout each fill cycle until the target weight is reached.
The advantages of adapting an automatic control system to an electronic laboratory balance for the purpose of accurately dispensing minute quantities of powdered material have been recognized for some time. Various attempts have been made to accomplish this with limited success due to the inherent operating characteristics of the typical electronic laboratory balance. In order to attain the high accuracy required, the electronic weight signal undergoes extensive electronic and digital filtering to reduce the influence of ambient vibration on the reported weight. The filtering process results in significant signal delay and relatively infrequent update of the registered weight. These two factors make it virtually impossible to utilize a typical feedback control system to regulate the flow of powder onto the weighing device as in earlier U.S. Pat. No. 4,431,070. A precision powder dispensing system requires a specific weight to be filled as well as. an accurate determination of the filled weight. Since the accurate determination of filled weight is inherent in a typical laboratory
problem revolves electronic balance, the filling around controlling the powder feeding system in a manner that will produce a spebific final weight within an acceptable tolerance
It is therefore proposed to provide a system which will greatly increase both the speed and accuracy of filling of powdered materials notwithstanding variations in flowability and particle size, is simple to operate and requires a minimum of operator assistance and is self-correcting to the extent of being substantially automatic.